1. Field of the Invention
The present invention relates generally to an active suspension system for an automotive vehicle for absorbing and damping vibratory energy to achieve a high level of riding comfort and driving stability and, more particulary, to a pressure control valve unit to be employed in the active suspension system for adjusting suspension characteristics depending upon vehicular driving conditions and the type of vibrations encountered.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987, commonly assigned to the assignee of the present invention, discloses a typical construction of an active suspension system applicable for an automotive vehicle. The disclosed active suspension system includes a hydraulic cylinder disposed between a vehicular body and a suspension member rotatably supporting a vehicular road wheel. The hydraulic cylinder includes a thrusting piston which divides the interior space of the cylinder into a working chamber and a reference chamber. The working chamber is connected to a hydraulic circuit including a fluid pressure source unit and a pressure control valve. The pressure control valve incorporates an electrically operable actuator which controls valve position for adjusting the operational mode of the active suspension system for absorbing vibratory energy which causes relatively high frequency and small magnitude vibrations to the vehicular body resulting in a `rough ride` feeling. The active suspension system described above also has an operational mode for damping vibratory energy which causes relatively low freqeuncy, large magnitude vibrations to the vehicular body which result in vehicular body attitude changes. The above described operational modes are neseccary in order to accomplish both high level riding comfort and driving stability.
For this purpose, the pressure control valve of the active suspension system includes a pilot pressure operated valve member which is movable between a pressure increasing position to increase fluid pressure in the working chamber, a pressure decreasing position to decrease fluid pressure in the working chamber and a pressure holding position to maintain a constant fluid pressure in the working chamber. The valve member is active for establishing fluid pressure balance between the working chamber and the reference chamber while in the pressure holding position so that the unsprung mass induced mode of the vibratory energy is converted into fluid pressure and the converted fluid energy is effectively absorbed.
The active suspension system includes an electronic or electric control unit designed to receive one or more suspension control parameters from one or more sensors. The control unit detects sprung mass induced vibratory energy, having relatively low frequency and large magnitude, and produces a suspension control signal to be supplied to the pressure control valve for operating the valve member between the pressure increasing position and the pressure decreasing position in order to suppress vehicular body attitude change which is caused by sprung mass induced vibratory energy.
Another construction of a pressure control valve unit has been disclosed in U.S. Pat. No. 4,801,155, issued on January 31, 1989, to Fukushima et al, which has been commonly assigned to the assignee of the present invention. The shown pressure control valve unit has a valve construction different from the foregoing prior art. However, the disclosed pressure control valve performs substantially equivalent pressure adjusting operations as that done by the foregoing prior art.
Such prior proposed active suspension systems are generally effective and successful in achieving substantially or satisfactorily high level vehicular riding comfort and driving stability. However, on the other hand, when a vehicle travels over a substantially rough road causing bottoming, the above described systems tend to cause too rapid variation in the fluid pressure in the working chamber and/or in the pressure control valve to effectively absorb this type of vibratory energy. When this happens, an extraordinarily high pressure is induced within the hydraulic circuit. In view of such possibility, the components of the active suspension system have to be designed so as not to break or become damaged even at such extraordinarily high pressures. Building a hydralic circuit and valve which can sustain such pressures clearly adds to the cost of producing a vehicular suspension system.